If you don't have any luck with repairs and reach a point where you have nothing to lose you can try to dismantle and re-assemble. It is possible with a lot of patience. You need to very carefully pick off the dustcap and put it to one side for later. Next you peel off the outer surround from the chassis. The card ring will de-laminate and be very easy to re-stick later. Next, and most difficult, you need to loosen the rear spider's grip on the rear chassis platform. You will need to source some vicious solvent like Inhibisol or methyl-ethyl-ketone (MEK). Do this outdoors. Apply the solvent to the outer edge of the cloth suspension, aka spider, and after about 10-20 minutes start to pick at the outer edge. Keep applying more solvent. Eventually the spider will start to lift and then you can peel it free from the chassis. All adhesives are weaker in peel. Peeling will minimise the damage. You need to reuse everything so take your time to avoid tearing. If you want, you can unsolder the leadouts and remove the whole cone assembly to repair the split properly. If not you need to re-assemble, re-centralising as you go.

Assuming the magnet has not gone off centre on the chassis from the blow and the top plate and pole are still concentric, you will need some 0.010" (ten thou) thick plastic shim material (shirt collar stiffener?). Clear out the magnet gap with folded masking tape and insert the shim all around the gap with no overlap. You can then re-insert the diaphragm assembly. Try it dry for fit and practice then apply solvent adhesive to the spider and water based adhesive to the card ring, then go again for real. Leave overnight then remove the shim and test for rubbing. Reglue the dust cap if all is OK.